An example of the construction of a prior art spread spectrum modulator is shown in FIG. 1(a). This prior art spread spectrum modulator includes a spreading signal generator 33 which supplies a spreading signal to a multiplier 31. The multiplier receives the spreading signal and an information signal 100 and combines them to form a modulated output. A bandpass filter 32 receives the modulated output of the multiplier and has the function of limiting the signal bandwidth prior to transmission of spreading modulation signal 110 so as to reduce, as much as possible, interference coupled onto an adjacent frequency channel which might be used by other transmitters or communication systems.
The operations of the above-described spread spectrum modulator will now be described. An information signal 100 having a limited bandwidth such as shown in FIG. 1(b) is received at the input to the multiplier 31 of the spread spectrum modulator. Spreading signal generator 33 generates a periodic spreading signal which has a very broad frequency spectrum such as shown in FIG. 1(c). As illustrated in FIG. 1(c), the frequency spectrum of the spreading signal can be expressed as the sum of discrete line spectra, which correspond to sine wave signals in the time domain. As a result of multiplying the information signal 100 by the spreading signal a modulated output signal is produced which has a very broad frequency spectrum as illustrated in FIG. 1(d). The modulated output is input to the bandpass filter 32 which then limits its bandwidth, so as to output a modulated signal having a frequency spectrum as illustrated in 1(e).
FIG. 2 illustrates an example of the construction of a prior art transmitter containing a prior art spread spectrum modulator of the type as illustrated in FIG. 1. The prior art transmitter incorporates a microphone 41 for transducing a voice signal 200 into an analog electrical signal, a coder 42 for converting the analog signal into a digital information signal, and a spread spectrum modulator 43 of the type as illustrated in FIG. 1 coupled to receive the digital information signal. The digital information signal is received at the input to the multiplier 48 of the spread spectrum modulator 43. Spreading signal generator 50 generates a periodic spreading signal which is also input to multiplier 48. The modulated output of multiplier 48 is input to the bandpass filter 49. A frequency converter 44 and an additional bandpass filter 45 are coupled to the output of the spread spectrum modulator 43 for respectively converting the modulated signal to transmission frequency and limiting its transmission bandwidth. An amplifier 46 is used to amplify the output of the bandpass filter 45. Antenna 47 is used to transmit the output signal of the amplifier 46 over the airwaves.
The operation of the prior art transmitter will now be described. Microphone 41 transduces a voice signal 200 to an analog electrical signal and inputs it to the coder 42. The coder 42 converts the analog signal into a digital signal, and depending on the case, may be used to digitally process or compress the signal. The coder 42 then outputs the resulting digital signal to spread spectrum modulator 43 for modulation thereof. After modulation, the frequency converter 44 shifts as denoted by .DELTA.f, in a manner as illustrated in FIG. 2(b), the baseband output of the spread spectrum modulator 43 to a wireless transmission frequency, which results in a modulated transmission signal having a frequency spectrum such as shown in FIG. 1(e), except for being shifted up to the transmission frequency. While a negatively shifted signal frequency spectrum appears in FIG. 2(b), only the positive portion of the frequency spectrum is transmitted.
The bandpass filter 49 used in spread spectrum modulator 43 generally does not have a sharp cut-off characteristic which would eliminate signal components which lie outside of the assigned transmission frequency bandwidth. Therefore, the prior art transmitter incorporates an additional bandpass filter 45 to eliminate frequency components in the output of frequency converter 44 which lie outside of the assigned transmission band. In this way, the level of interference coupled between signals transmitted on adjacent frequencies can be kept within acceptable limits.
The prior art spread spectrum modulator and transmitter which are illustrated in FIGS. 1 and 2 rely on bandpass filters to minimize the interference coupled onto adjacent signal frequencies. Since the frequency spectrum of the input signal to the bandpass filter 32 has a shape as illustrated in FIG. 1(d), and the frequency spectrum of the output signal must have a shape as shown in FIG. 1(e), the bandpass filters 32 and 45 must have sharp cut-off characteristics.
In order to implement a bandpass filter in hardware which has a sharp cut-off characteristic, the scale of the hardware must be increased. On the other hand, if such bandpass filter is to be implemented in software, the calculation quantity must be increased beyond acceptable limits. As a result, with the prior art transmitter and modulator, a design point must be reached which either results in unsatisfactory coupling of interference onto adjacent channels, or in modulator systems which require expensive, bulky filter hardware and/or additional processing power to appropriately band-limit signals for transmission.
In addition, in prior art spread spectrum transmitters, because the incorporated bandpass filter 32 of the spread spectrum modulator 43 has unsatisfactory cut-off characteristics, an additional bandpass filter 45 must be provided to reduce the interference coupled onto adjacent frequency channels to within acceptable limits.
It is therefore an object of the present invention to provide a spread spectrum modulator which does not require a bandpass filter, while eliminating the interference coupled onto an adjacent signal frequency.
It is a further object of the invention to provide a spread spectrum modulator which permits a reduction in the scale of hardware and/or the calculation quantity of software.
Still another object of the invention is to provide a spread spectrum modulator for incorporation into a spread spectrum transmitter which produces a modulated signal output that is free of interference components that may be coupled onto adjacent signal frequencies, such that the spread spectrum transmitter does not require a bandpass filter.